A blog about the history, current state, and the future of the electric guitar and tube-based amplifier combo and how this de rigueur musical instrument that matured during the Rock N' Roll-era continues to both evolve and refuses to die.

Sunday, March 29, 2015

Have you ever wondered why there are no commercially
produced hi-fi audio power amplifiers using the 6V6 output vacuum tubes despite
its widespread use in electric guitar amplifiers?

By: Ringo Bones

Despite its widespread use in the “guitar world”, the 6V6
vacuum tube is probably the only widely available vacuum tube – there’s even an
excellent sounding version in current production by Electro-Harmonix – that no
commercially produced hi-fi audio power amplifier uses as its output power
tube. While its closest equivalent – the older venerable 6L6 is widely used in
both electric guitar and hi-fi amps, the 6V6 seems to have stuck in the “guitar
world” despite the existence of DIY hi-fi literature dating from the 1950s -
i.e. 1950s era RCA application notes - with circuit diagrams of audio power
amps using the 6V6 vacuum tube.

The 6V6 is a beam power tetrode that was introduced by RCA
back in mid 1937 and still in current use, especially in electric guitar
amplifiers. Similar to its predecessor – the 6L6 which was introduced back in
1936 – the 6V6 was far more widely used during its heyday – i.e. from 1945 to
the early 1970s when solid-state transistor became reliable and commercially
viable enough for household audio amplification purposes. In comparison, the
6L6 produced more output power than required by general use consumer electronic
devices right after World War II with corresponding power and circuit
requirements.

The lower powered 6V6 was better suited for the average home
use and became common in the audio output stage of “farmhouse” table top radios
(Stromberg Carlson?) where power pentodes such as the 6F6 had previously been
used. The 6V6 requires less heater power and produced less distortion than the
6F6 while yielding a higher output in both single-ended and push-pull
configurations. Additionally, before the transistor era, the 6V6 has
applications in the automotive and portable radio market. In an audio output
stage, a single 6V6 can be used to produce 5-watts continuous power and a
push-pull pair for about 14-watts with the higher output requiring a larger,
more expensive output power transformer with grain oriented core laminations
for better efficiency.

The 6V6 was first introduced in both metal and shouldered glass
tubes. RCA was promoting the superiority of their metal tube designs during the
second half of the 1930s and this tube having been introduced during that
period was produced in large quantities in this format. Other vacuum tube
manufacturers also produced the 6V6 in glass tubes which were commonly found in
radios not made by RCA. By 1940, the 6V6 was mostly being produced in smaller
“GT” glass envelope and later the 6V6GTA was introduced which has a controlled
warm-up period.

Generally speaking, 6V6 output power vacuum tubes are sturdy
and can be operated beyond their published specifications. The 6P6S – which has
poor tolerance for out-of-spec operation versus most American and Western
European made 6V6 variants – is an exception. Because of this, the 6V became
very popular for use in consumer market musical instrument amplifiers
particularly combo style electric guitar amps such as the Fender Champ which
uses the 6V6 in a single-ended configuration. While the Gibson GA-40 and the
Fender Princeton Reverb and the Fender Deluxe Reverb amps which run a push-pull
pair of 6V6s at 410-VDC plus on the plates. This market allows Mainland
Chinese, Slovak and Russian vacuum tube factories to keep the 6V6 in production
to this day. Because of the relative similarity in voltage and other
characteristics between the 6V6 and the "more popular" EL84 / 6BQ5 power pentode
vacuum tube, several electronics and musical instrument companies have
developed adapters to allow an amplifier with 6V6 octal sockets to accept the
miniature noval pinned EL84. However no reverse adapter has been made or
developed to be commercially available to allow an EL84 tubed amp to accept 6V6
output power vacuum tubes.